The present invention relates a valve train for an internal combustion engine, and more particularly to a valve train in which sliding portions of a camshaft and valves and/or counterparts thereof are coated with a hard carbon film (coating) such as a diamond-like carbon (DLC) film performing an excellent lower friction through a specified lubricating oil (lubricant).
Global environmental problems, such as global warming and ozone layer destruction, have been coming to the fore. It is said that the global warming is significantly effected by CO2 emission. The reduction of CO2 emission, notably the setting of CO2 emission standards, has therefore become a big concern to each country.
One of challenges to reduce CO2 emission is to improve vehicle fuel efficiency, and the sliding members of a vehicle engine and a lubricating oil thereof are largely involved in the improvements in vehicle fuel efficiency.
The material for the sliding members is required to have an excellent wear resistance and low friction coefficient even when heavily used as a sliding member of an internal combustion engine under a severe frictional and wearing condition. Lately, there have been developed the application of various hard film materials and the application of a locker arm with a build-in needle roller bearing, with respect to a follower member such as a valve lifter and a lifter shim.
In particular, a diamond-like carbon (DLC) material is expected to be useful as a coating material for the sliding member, because the DLC material provides a lower friction coefficient in material in the atmosphere and/or non-oil condition than that of another wear-resistant hard coating (film) material such as such as titanium nitride (TiN) and chromium nitride (CrN).
There are the following approaches to improving the vehicle efficiency in terms of the lubricating oil: (1) to decrease the viscosity of a lubricating oil in the sliding mechanism, thereby reducing viscous resistance in hydrodynamic lubrication regions and sliding resistance in the engine; and (2) to mix a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication. Heretofore, researches have been made on an organomolybdenum compound, such as molybdenum dithiocarbamate (MoDTC) or molybdenum dithiophosphate (MoDTP), for use as the friction modifier and show that the lubricating oil containing such an organomolybdenum compound is effective in reducing friction when used for the steel sliding members.
Documents disclosed in Japan Tribology Congress 1999.5, Tokyo, Proceeding Page 11–12, KANO et.al. and in World Tribology Congress 2001.9, Vienna, Proceeding Page 342, KANO et.al. have reported friction characteristics of the DLC material and the performance of organomolybdenum compound used as a friction modifier. Further, Japanese Published Utility Model Applications No. 5-36004 and No. 5-42616, and Japanese Published Patent Application No. 8-14014 have proposed various improvements in an engine valve train.